In recent years it has been stated that generation of greenhouse gases leads to global warming and that further increase in greenhouse gas production will further accelerate global warming. Since CO2 (carbon dioxide) is identified as a main greenhouse gas, CCS (carbon capture and storage) is considered one potential major ways to reduce the release of greenhouse gases into the atmosphere and to control global warming. In this context CCS can be defined as the process of CO2 capture, compression, transport and storage. Capture can be defined as a process in which CO2 is removed either from the flue gases after combustion of a carbon based fuel or the removal and processing of carbon before combustion. Regeneration of any absorbents, adsorbents or other ways to remove CO2 of carbon from a flue gas or fuel gas flow is considered to be part of the capture process. There are several possible approaches to CO2 capture in power plants, e.g. in coal fired steam power plants, gas turbine or combined cycle power plants. The main technologies under discussion for CO2 capture are so called pre-combustion capture, oxyfiring, chemical looping and post-combustion capture.
Pre-combustion carbon capture involves the removal of all or part of the carbon content of a fuel before burning it. For natural gas, this can be done by reforming it with steam, followed by a shift reaction to produce CO2 and hydrogen. The CO2 can be captured and removed from the resulting gas mixture. The hydrogen can then be used to produce useful energy. The process is also known as synthesis gas or syngas approach. The same approach can be used for coal or any fossil fuel. First the fuel is gasified and then treated in the same way as natural gas. Applications of this approach in combination with IGCC (Integrated Gasification Combined Cycle) can be envisioned.
Oxyfiring (also known as oxyfuel firing or oxygen combustion) is a technology that burns coal or other fossil fuel in a mixture of oxygen and recirculated CO2 rather than air. It produces a flue gas of concentrated CO2 and steam. From this, CO2 can be separated simply by condensing the water vapor, which is the second product of the combustion reaction.
Chemical looping involves the use of a metal oxide as an oxygen carrier, typically a metal oxide, which transfers oxygen from the combustion air to the fuel. Products from combustion are CO2, reduced metal oxide and steam. After condensation of the water vapor, the CO2 stream can be compressed for transportation and storage.
The CCS technology currently considered closest to large-scale industrial application is post combustion capture combined with compression, transportation and storage. In post-combustion capture the CO2 can be removed from a flue gas. The remaining flue gas can be released to the atmosphere and the CO2 can be compressed for transportation and storage. There are several technologies known to remove CO2 from a flue gas such as absorption, adsorption, membrane separation, and cryogenic separation.
Known technologies for CO2 capture and compression can involve relatively large amounts of energy. There are many publications on the optimization of the different processes and the reduction of the power and efficiency penalty by integrating these processes into a power plant.
For CCS with post combustion capture, the CO2 capture and the compression of CO2 for further processing, i.e. transport and storage, can lead to a decrease in the net power output reduction of a plant relative to a known plant without CCS.
EP1688173 gives an example for a post combustion capture and a method for the reduction of power output penalties due to CO2 absorption, respectively the regeneration of the absorption liquid. Here it is proposed to extract steam for regeneration of the absorbent from different stages of the steam turbine of a power plant to minimize a reduction in a turbine output.
In the same context, WO2007/073201 suggests to use the compression heat, which results from compressing the CO2 stream for regeneration of the absorbent.
These methods aim to reduce the power requirements of specific CO2 capture equipment, however the use of the proposed CO2 capturing method will always result in a significant reduction of the plant capacity, i.e. the maximum power a plant can deliver to the grid.
An attempt to mitigate the impact of CO2 capture on the plant performance by increasing plant flexibility is disclosed in EP0537593. EP0537593 discloses a power plant that utilizes an absorbent for CO2 capture from the flue gases, where the regenerator can be switched off during times of high power demand and where the CO2 capture continues by use of absorbent stored in an absorbent tank during these times. EP0537593 discloses a simple on/off mode of one power consumer of the CO2 capture equipment. It adds only very little operational flexibility at relatively high cost.
Frequency response is an important issue for power plant operation and also has to be considered for plants with CO2 capture and compression. EP0858153 discloses basic principles of frequency response, in which a grid has a grid frequency, which fluctuates around a nominal frequency. The power output of the power plant can be controlled as a function of a control frequency, in such a matter that the power output can be increased when the control frequency decreases below the nominal frequency, and the power output can be decreased when the control frequency increases beyond the nominal frequency. The grid frequency can be continuously measured. EP0858153 discloses a method to average the grid frequency and to use the measured grid frequency as the control frequency, however it is limited to known control mechanisms of a gas turbine power output control. To enable response to under-frequency events, plant normally have to operate at part load.